Oh Emma I don't envy your situation. But as an outsider I am seeing a bit of light peaking around the corner from the other end of your tunnel. You likely have a long way to go before you reach it but I am seeing the first little glimmers. Breathe. Just keep breathing. Every piece of information will help you build your case and formulate a plan to help your DS get what he needs. Just keep breathing...
The no reading tutor is not good for your DS but provides some very good info for working on a solution. That's the irony of these situations - you need to uncover the holes (pardon the pun) in order to know what to address, but each hole you discover is maddening. I know when I was in the worst of our battle I would uncover things like that and lose my mind. I would scream and yell, cry, pace the floor in the middle of the night and then eventually have some sort of an epiphany that would allow me to use the information to open the next door. It is exhausting - physically, emotionally, psychologically, financially - but sadly it is often our reality. This is why breathing is so important. Breathe again.
So how do you use this new information? Do just what the SW said - get "the appropriate educational professional" to "outline the needed adjustment". I think you were brilliant to use the DSM/neurological approach to capitalize on the "medical final say" aspect of your parenting plan. Now you follow up with that. Find the dyslexia expert that I mentioned in an earlier post - OG, Wilson, Lindamood Bell, etc - and have them write up a detailed plan of what a child with your DS's diagnosis *should* be receiving. My guess is it will include daily intervention (or at least multiple times a week) by an appropriately certified tutor/ sped Ed teacher. Have them "outline the needed adjustment". Not you - them. And make sure their letter is signed with as many letters after their name as possible.
An OT eval outlining what he should be receiving for the dysgraphia would also be good. Again with details and as many letters after their name as possible. Maybe try to get an AT evaluation on your own if the district or school hasn't done one. My DD's entire education is reliant on her AT - predominantly an iPad. CoWriter on an iPad Air allows her to move between keyboarding and voice to text, she listens to audio books at a much higher level than she could possibly read on her own, uses math apps, etc. It is an essential tool for her. If your DS doesn't have one in his program you need someone - also with a lot of letters after their name - writing a letter or report explaining why he needs one and "outlining the needed adjustment".
And a psychologist to weigh in on the anxiety, depression or whatever he is formally developing as a result of his LD's not being properly supported in this school environment. Again lots of letters after their name as they "outline the needed adjustment". Notice a pattern here? This is once again a situation of lather, rinse, repeat. Your GAL's SW has told you what you need to provide. Now it's a matter of locating the appropriate people and getting their written input. Also try to see if there is a neurologist in your area with an LD specialty. It would bolster your medical argument and firmly allow you a seat at the table if you could get an MD weighing in to say that your DS needs better/ more appropriate interventions.
I'm not going to lie to you - this will be exhausting and expensive. But it's also what your DS needs. Your district will likely support your argument because they are paying a hefty amount for a program that is not meeting your DS's needs. It's great you will be attending the IEP meeting and will be able to present this info there. I'm a bit confused about where FAPE/IDEA fit here. They may actually apply since your ex sued and the district is paying for the school. Your whole medical vs educational final say situation really complicates things though. I'm glad you found a hook to fight for these needed services. Hopefully the GAL will get it and make appropriate recommendations and hopefully the district will cause changes to happen. I assume you could get a mediation session scheduled earlier while you wait the 2 years for a court date. Maybe that could get the ex, the school and the district to agree to alter his program. But you need that detailed expert input first. I wonder if he needs a placement similar to my DD at a spec Ed school. The district is already paying for this school - if only they could pay for a more appropriate one instead. It really is madding!
I hope some of this helps. I feel for your DS. I often wonder what would have happened to my DD if I hadn't fought as hard as I did to get her what she needs. I see your DS's situation and know it was worth everything I had to go through. Your ex being in denial about the situation is really making your DS's already difficult situation so much harder. Thank goodness he has you. Keep fighting for him. Keep your eye on the prize. And of course keep breathing!
Sending hugs and support your way!